The late Robin Williams even brought improv to his serious roles as seen in Good Will Hunting
To make a great film, you need a bunch of things to all work together seamlessly. These things include actors' performances, directors and script writing.
There are some actors who like to take on a little more than everyone else, though.
Improvisation isn't easy, but when done right, it brings something extra to a film.
No matter how good an actor may be, reading from a script doesn't come naturally like improvisation does. It's just inherently more natural.
Good Will Hunting has won or been nominated for numerous awards, one of which would be The Academy Award's Best Supporting Actor which was won by the late Robin Williams. Williams, being a comedian, is known for his ability to improv, and he doesn't disappoint in this film.
One memorable scene from the film was when Matt Damon's character Will is in therapy with Robin Williams' character Dr. Sean Maguire. Williams talks about his wife's little idiosyncrasies, including how she farts in bed.
The whole thing was improve, and the seemingly out-of-control laughter by Matt Damon is his real reaction, as he was taken completely by surprise. He wasn't the only one. Damon swears that if you look closely, you can see the camera swaying up and down slightly due to the cameraman laughing.
Williams also improvises the last line of the film, while Ben Affleck and Minnie Driver each made significant contributions to their characters.